1. Technical Field
The invention relates to a fail-safe device particularly, but not exclusively, for use in reinforced concrete structures.
2. Background Art
Buildings and other structures are generally designed and built for static situations on the basis of the minimum required strength of their constituent components. As a result, in order to ensure that such minimum design criteria are easily met, many components are over-designed or over-specified and there is little or no perceived penalty in installing stronger components than are actually required.
However, increasingly attention is being paid to the design of structures in areas of the world which are prone to earthquakes and, in such areas, over-design or specification can bring with it inherent problems. For instance, in the past much attention was brought to bear on designs for so-called xe2x80x9cearthquake-proofxe2x80x9d structures capable of withstanding seismic activity. Unfortunately, as recent experience in Kobe has shown, there really is no such thing as an xe2x80x9cearthquake-proofxe2x80x9d structure and, when a building finally does give-way it can often occur in an unpredictable and unsafe manner leading to much loss of life.
Engineering design standards in areas prone to earthquakes are still in a state of flux, but a key element in modern design approach is to accept that some earthquakes will be too powerful to withstand.
A European standard, known as Eurocode 8 has been directed toward the issue of building designs in earthquake areas. According to Eurocode 8, a set of prerequisites regarding the mechanical properties of reinforcement bars used in reinforced concrete are detailed. The aim of Eurocode 8 is to maximize safety for building users. Such safety maximization is attained by ensuring that the building will respond in a ductile fashion to seismic activity.
Whilst this Eurocode 8 is in existence, there are a number of problems in implementing it. Earthquakes vary enormously in their magnitude. To adopt the same design methods as are used to accommodate gravity, wind, etc., for dealing with earthquake loads would lead to over design. Since all structures need to be built to an economic level, over-design is simply not practical. Also, if the structure is built to have what might be regarded as an elastic response (i.e. able to take the load and recover fully) then the large values of acceleration which could result in practice from such design methodologies could in themselves endanger lives and cause extensive non-structural damage.
Earthquake-resistant structures are usually designed to respond in a non-linear fashion so that below certain seismic load levels, the structure behaves elastically, but when the load goes above a given value, the structure is designed to deform inelastically without significant loss of strength. Such a design is more economical than a fully elastic approach and allows for seismic loads which are higher than those originally predicted during design.
The capacity of a structure to deform without significant loss of strength, know as ductility, is of paramount importance in earthquake engineering. In general ductility is defined as the ratio of deformation at a given response level to deformation at yield response. Thus, its definition can be applied at section, element or structure level.
Concerning structural ductility, earthquake resistant structures generally now follow a xe2x80x9ccapacity design philosophyxe2x80x9d in which the structure is viewed as having two different types of zones, i.e. zones which are xe2x80x9cdissipativexe2x80x9d and zones which are xe2x80x9cnon-dissipativexe2x80x9d. The dissipative zones are those which are responsible for the mobilization of the desired failure mode, chosen to maximize overall energy absorption capacity and avoid collapse. All other zones are considered non-dissipative. The dissipative zones must be dimensioned first and carefully detailed to possess maximum ductility. Next, the amount and sources of xe2x80x9coverstrengthxe2x80x9d are assessed. Such sources of overstrength include: higher concrete compression strength; confinement; larger area of steel due to the availability of bar diameters; higher yield strength of steel; and strain hardening.
The non-dissipative parts of the structure are then designed to withstand forces which are consistent with the strength of dissipative parts, including sources of overstrength. In this way, the structure can be rendered less sensitive to the characteristics of the input motion, since it can only respond in the ductile mode that was envisaged in the design phase, resulting in increased control to seismic response.
To summarize the above, it has been found that instead of relying upon static design, it is better to limit damage by designing in yield, so allowing structures to flex and compensate in a predictable predetermined way to minimise damage and loss of life rather than risking catastrophic failure of the whole structure and the lives of all the occupants.
Unfortunately, up until now implementing this design ethos has been made very difficult, if not impossible, due to the fact that the reinforcing bars (rebars) used in reinforced concrete structures are obtainable from a wide variety of sources and manufactured to wide tolerances. This means that although it is supplied to conform to minimum strength specifications, these minimum margins may be exceeded by a considerable and highly variable margin.
Accordingly, it is an aim of preferred embodiments of the present invention to provide a fail-safe device for use in reinforced concrete structures which is designed to yield under closely specified predetermined conditions to enable the implementation of fail-safe structures.
According to a first aspect of the invention, there is provided a fail-safe device for use in a reinforced concrete structure, the device comprising an elongate link, for connection with a length of reinforcing bar, wherein the link is designed to yield within predefined tolerances under certain limit load conditions.
Preferably, the limit load conditions are brought about by seismic events such as an earthquake or may be due to sudden impact, explosions or the like.
The device may form part of a reinforcing bar or may be a separate unit with first and second ends for respective connection with first and second lengths of reinforcing bar.
Preferably, the link has a transverse cross sectional area which is greater at end regions than at a region between those end regions.
Preferably, the link has a waisted appearance such that it tapers from end regions thereof towards a middle region.
Preferably, the link is formed of a high tensile strength ductile material, such as a high strength alloy steel.
Preferably, where the device is inserted within a length of reinforcing bar or joined to first and second lengths of reinforcing bar, connections between the device and the bar are full strength connections.
By a full strength connection, it is intended to mean that the connection itself between device and reinforcing bar is at least as strong as the reinforcing bar.
Preferably, the full strength connection is achievable by means of providing end regions of the link with a threaded region and providing end regions of the reinforcing bar with a rolled thread and coupling threaded regions of the link and reinforcing bar together by means of an internally threaded sleeve, wherein a thread minor diameter of the reinforcing bar is arranged to be less than a nominal diameter of the bar but a thread major diameter is arranged to be greater than the nominal diameter of the bar. Such a connecting system is described in PCT application number PCT/GB95/00309, as applied for in the name of CCL Systems Limited.
It is most important that connections between reinforcing bar and the device are made by means of such full strength couplings since it is most important that the link itself should give way under limit load conditions, rather than the coupling between link and reinforcing bar.
Preferably, the tensional force required to cause failure of the link is determined by tensile test measurements of a sample of a material from which the link is manufactured.
Preferably, the link is provided with a finely ground finish and this finish determines tolerances in the tension applied within which yield will occur.
Preferably, the device further comprises a coating or encapsulating layer to protect at least part of the link against a damage. The coating or encapsulating layer may be of a solid substance arranged to provide protection against damage such as may be caused by corrosion, impact, or abrasion.
Preferably, the coating or encapsulating layer is of a resinous substance.
Preferably, the coating or encapsulating layer is separated from the link by a debonding agent.
The fail-safe device is preferably provided with a strain gauge attachment, said attachment including connections to external instrumentation for assessing the stress or strain on the device. The strain gauge attachment may be associated with the device by any suitable means, such as using an adhesive to bond the attachment to the waisted area of the link.
According to a second aspect of the invention, there is provided a structure including one or more fail-safe device in accordance with the first aspect of the invention.
Preferably, the fail-safe device is provided in one or more beams and/or columns of the structure.
According to a third aspect of the invention, there is provided a method of manufacturing a fail-safe device, the method comprising: taking a bar of high tensile strength material and cutting that bar to a predetermined length; taking the predetermined length of the bar and turning a central region thereof in order to provide that central region with a reduced diameter; applying a debonding agent to the reduced diameter region; and encapsulating the central region with a protective substance.
Preferably, the remainder of a bar of material from which the fail-safe device is formed is retained for future reference.
Preferably, the diameter of the central region is determined by carrying out controlled tests on the parent material so as to determine a precise diameter required for a given yield strength.
Preferably, end regions of the device are provided with means for connecting them with one or more reinforcing bars.
Preferably, the means for connecting comprises threading the end regions of the device.